This invention relates to rotary mechanisms for cutting and/or sealing web materials. It is particularly concerned with the cutting and/or sealing of a continuously moving web by apparatus comprising counter-rotating rotors between which the web passes.
Heat sealing operations in such apparatus relies on the web material being momentarily engaged between opposed faces of the rotors as those faces come into juxtaposition and apply heat and pressure to the web material. If the material is not also to be severed by the heat and pressure, the contact pressure can be critical. To safeguard against inadvertent severing of the material it can be arranged that the mutually opposed faces are held apart at a small spacing as they come together. For this purpose it is known to provide contacting bearer elements standing slightly proud of the sealing faces on the rotors adjacent the web-gripping faces, as described in WO95/01907 (U.S. Pat. No. 5,548.947), that maintain a gap between those faces. For high speed running, these bearer elements can extend circumferentially in front of the sealing faces in the direction of rotation, to help ensure that dynamic disturbances from their initial contact do not affect the sealing process adversely.
For sealing web materials such as thermoplastic-coated paper, it is known to apply a crimp to seal the material more securely. Thus, the rotor sealing faces are formed with complementary arrays of V-grooves, running in the circumferential direction over the faces. If the complementary arrays are not closely aligned, however, a non-uniform spacing appears with an increased gap on one face of each V-groove and a reduced gap on the opposite face. This asymmetry produces a resultant lateral force component tending to limit such misalignment, but the higher pressure where the gap is reduced can damage the web material.
If the V-groove arrays are extended over the bearer elements, they will tend to correct any misalignment as they come into engagement, before the sealing faces engage the web material. However, as the faces of the bearer elements make direct contact, there are then very high local peak pressures experienced which can give rapid wear. Such wear may occur because there may be a relative misalignment between the pairs of bearer faces at opposite axial ends of a sealing face, since even a very small dimensional difference will prevent the grooves of both pairs of bearer faces meshing fully. Close control of dimensional tolerances can ensure that misalignment from this source will not adversely affect the web material, but larger misalignments. can be created by disturbing forces, such as differential thermal expansion effects. In that case, because the misalignment will only be reduced to the extent that the disturbing forces are balanced by the resultant lateral force on the bearer element grooves, their meshing may still leave significant non-uniformity in the spacing between the crimping grooves on the sealing faces.
According to a first aspect of the present invention, there is provided a rotary mechanism comprising at least one pair of rotors mounted for counter-rotation on parallel axes each having radially projecting arms with cutting and/or sealing means on radially outer faces of the arms to cut and/or seal web material between juxtaposed outer faces of the arms of said pair of rotors, the arms being provided with bearer elements arranged to contact each other when the faces are brought together and set a spacing between said juxtaposed faces, the cutting and/or sealing means of each outer face being located between first and second said bearer elements of the respective rotor arm, the cooperating first bearer elements at one end of the juxtaposed faces comprising means locating the arms relative to each other in the direction of the axes of rotation of the pair of rotors, and the cooperating second bearer elements at the other end of said faces permitting relative movement in said axial direction.
In this way it is possible firstly to reduce wear through relative misalignment between the bearer elements at opposite ends of a pair of sealing faces because the second pair of bearer elements are not constrained laterally by their engagement. In addition, however, it allows the adoption, in a preferred form of the invention, of an arrangement in which each said cooperating pair of first elements have on each element a pair of locating faces extending in substantially radial planes, said faces of one element of the pair being engaged between said faces of the other element of the pair.
While disposing said pairs of faces precisely in radial planes provides a form of engagement that positively precludes lateral misalignment, in practice the same effect can be achieved if they are inclined to some degree from the true radial direction.
Preferably, said pairs of faces are formed as mating male and female elements occupying only a minor part of the width of their bearer faces, the remaining parts of which will provide radial location setting the spacing between the rotor arms. In that case, said male and female elements may have respective radially outer and inner faces which are maintained out of contact with each other.
For effective use of the bearer elements to locate the arms of the respective rotors relative to each other, it is convenient to arrange that one rotor of the pair is resliently biased towards the other.
According to another aspect of the invention, there is provided a rotary mechanism comprising a pair of rotors mounted on respective carriers for counterrotation on parallel axes and each rotor having radially outer faces provided with cutting and/or sealing means to cut and/or seal web material engaged between said outer faces, one of the carriers being displaceably supported and being acted upon by biasing means to urge said outer faces into engagement, means being provided to limit the displacement of said one carrier by said biasing means, said biasing means being arranged to bear on said one carrier at a location substantially coincident with a plane containing the axes of the two rotors, and the displacement limiting means being provided by at least one stop element also substantially coincident with said plane of the rotor axes.
Such a mechanism can include with advantage the locating bearer elements of the first aspect of the invention so that transverse seals are made at intervals across a continuous tubular web and the web is severed simultaneously in the width of the seals in order to form a series of individual sealed packets.
If a rotary mechanism according to the first aspect of the invention is provided with web cutting means, the arrangement of the bearer elements is also able to offer an advantage as compared with the bearer elements referred to about complementary crimping V-grooves. As has already been explained, in the prior art rotors with grooved bearer elements, the V-grooves cannot be relied on to eliminate transverse misalignment in the mechanism and such misalignment will also cause a radial displacement when the bearer element faces engage each other. By the use of the bearer elements of the present invention, any correction of misalignment does not affect the relative radial positions of the rotor arms, so cooperating cutting elements on the juxtaposed outer faces of the arms remain unaffected.
This is particularly useful if the cutting elements are intended for crush cutting of the web material. In this cutting technique, the material is severed when it is nipped between a sharp-edged cutter element and an anvil element. The action requires a precise setting of the two elements relative to each other to ensure on the one hand that the cutting action is complete and on the other hand that the working life of the elements is not impaired by excess pressure loads.
It is usual to provide means for adjusting crush cutting elements in the radial direction to set them precisely and this is a requirement that can occur independently of the need to ensure the maintenance of the setting when axial misalignment of the rotor arm occurs.
In U.S. Pat. Nos. 5,211,096 and 4,785,697, GB 1469684 and 878556, and DE 4215196 and 4211187 crush-cutting means are provided on counter-rotating drums and adjustments are made by means of wedge elements which allow small controlled movements of the cutting elements to be made radially of their drums. There are limitations to these known adjustment mechanisms, however.
In particular, in the drums in which the crush-cutting elements and their adjustment means are deployed, the adjustment means occupy a relatively large space and they cannot easily be modified to fit a cutting and sealing mechanism comprising rotors provided with spaced, radially extending arms the outer ends of which carry the crush-cutting elements. Although operating conditions may dictate the more open configuration such a mechanism provides as compared with the generally cylindrical drums of these known forms of apparatus, the known adjustment means referred to above cannot be easily accommodated in such an alternative configuration and accessibility for adjustment may be compromised also.
Although some of the known crush cutting means referred to above have the mountings of the individual cutter and/or anvil elements set at fixed radial locations, others have some degree of resilient compliance in the mountings of the elements so that they can yield to pressure during the cutting operation. The yielding of the elements allows them to be set with a greater tolerance, but it introduces potential problems. For example, the mobility of the elements can be effective only to the extent that it is ensured that they remain free to move. Hostile operating conditions, eg. large temperature variations or a dusty or otherwise contaminated atmosphere, can easily lead to sliding bearings becoming locked fixed in their mountings. Also, if the cutter and anvil elements are not mounted true to each other, ie. if there is a run-out between the seatings of the cooperating elements along their length, the cutting pressures will differ along their length due to the tilting displacements of the elements.
According to a further aspect of the present invention, a rotary cutting mechanism is provided comprising at least one pair of rotors mounted for counter-rotation on parallel axes and each having radially projecting arms with cutting means on their axially outer ends, said cutting means comprising at least one elongate cutter element having a radially outer cutting edge and at least one cooperating elongate anvil element on the respective rotors, means for radial adjustment of the or each cutter element and/or the or each anvil comprising an elongate wedge member extending in the longitudinal direction of the or each said cutter element or anvil element and a radially inner face of said cutter or anvil element being engaged by an outer face of the wedge member, a generally radially directed face of the or each wedge member being inclined with respect to its direction of rotation, there being means for adjusting the wedge member relative to the associated cutter or anvil element transversely to said rotary axis and securing it in its position of adjustment, thereby to control the radial setting of the associated cutter or anvil element.
Such an adjustment mechanism can be accommodated in a relatively small space so facilitating a rotary cutter configuration in which the cutting means are carried by radially extending arms. Furthermore, if the wedge element adjustment means can be made accessible on the leading/trailing faces of the rotor arms it is possible for them to be easily reached when the apparatus is not running because the rotor configuration has free space in these regions that is not available in the rotor drum configuration.
In a preferred arrangement, the or each anvil element is adjustable by said wedge element radial adjustment means, and the or each cutter element is adjustable to bear against a fixed radial stop on its rotor arm.
Preferably, one rotor of the pair is held on mountings provided with resilient loading means that allow displacement of the rotor relative to the other rotor of the pair, eg. in the manner of a mechanism according to the first aspect of the invention. Thus, in contrast to prior art arrangements in which the individual cutting elements are made compliant, there can be resilient yielding between the rotors themselves when needed to prevent the contact pressure between them increasing unduly.
An example of the invention will be described by way of example with reference to the accompanying drawings.